4.1 -The History of Equine Evolution

To understand how to plan and create an organization to aid retired Thoroughbreds, one must appreciate and understand the horse species, Equus. This task is begun by examining the theories of equine evolution that have arisen over the past two-hundred years. The evolution of the horse has taken place during the Cenozoic era. Refer to Table 4.1 for clarification of this time period.

Table 4.1-Geologic Time

Era

Period

Epoch

Millions of Years Ago

Cenozoic

Quaternary

Holocene

Pleistocene

.5-present

2-.5

13-2

25-13

36-25

58-38

65-58

Tertiary

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Mesozoic

Cretaceous

Jurassic

Triassic

136-25

190-136

225-190

Paleozoic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

280-225

345-280

405-345

425-405

500-425

600-500

Precambrian

3980-600

Source: adapted from Sautter and Glover, 1981

The first theories of equine history were developed before Darwin’s contributions to the theories of evolution were disclosed in 1859. Richard Owen, the foremost zoologist in England in the 1840’s (Prothero and Schoch, 2002) examined several bone fragments and fossils found in various parts of his country. He concluded that the fossils could be classified according to the number of toes on each foot. Horses, rhinos, tapirs and hyraxes were all thrown into the odd-numbered toe group and given the name “Perissodactyla” (Prothero and Schoch, 2002, 198). The Perissodactyla order has since been redefined to include mammals in which “the axis of the foot passes through the middle toe of the foot” (Sautter and Glover, 1981, 27).

Charles Darwin and his associates entered the scene in the 1870’s recognizing that when the four available equine fossils were put into order based on epoch age, they represented a sequential progression leading to the modern Equus. The first of these fossils was called Hyracotherium (Prothero and Schoch, 2002). In 1873, Othniel Charles Marsh, a Yale Paleontologist in the United States, found fossil horses from the Eocene epoch of the Cenozoic era (Table 4.1) in the Rocky Mountains that were more complete than those found in Europe (Sautter and Glover, 1981). He concluded “the line of descent appears to be direct, and the remains now known supply every important form” (Marsh in Prothero and Schoch, 2002, 199). The “dawn horse” of the United States was called Eohippus (Prothero and Schoch, 2002). Although Marsh easily convinced Darwin’s associates that his theory was superior to theirs, the notion that equine evolution is direct and linear is now believed to be incorrect by most current evolutionary theorists (Prothero and Schoch, 2002).

It wasn’t until the 1930’s that Sir Clive Forster Cooper of the British Museum realized the European Hyracotherium and the American Eohippus were indistinguishable from one another. Since Owen proposed the name Hyracotherium thirty years before Eohippus was used, the first name became correct, although Eohippus is still used frequently (Waring, 1983, and Biracree and Insinger, 1982). However, in the late 1980’s, Jeremy Hooker of the British Museum discovered that both Owen’s Hyracotherium specimen and Marsh’s Eohippus specimen were not really horses at all. Therefore, the name of the “dawn horse” is still under revision (Prothero and Schoch, 2002).

In the 1920’s, Childs Frick, the son of a wealthy railroad and steel tycoon, decided to spend time and money collecting fossils of game animals from the Oligocene epoch through the Pleistocene epoch (Table 4.1). Morris Skinner, one of the men Frick hired to dig for fossils, took a special interest in equine fossils. However, Frick would not allow Skinner to publish any of his work on the evolution of horses. After Frick’s death in 1965, the American Museum of Natural History acquired his fine collection, which occupied seven entire floors, and Skinner was able to publicize his work (Prothero and Schoch, 2002). Skinner and his colleagues concentrated on exploring the time period between the Miocene and Pliocene epoch (Table 4.1), where there was much confusion concerning the extremely diverse group of three-toed “hipparions” (Prothero and Schoch, 2002, 209). Before Skinner’s time, all “hipparions” that had similar tooth structure were thrown into a single group called Merychippus. However, Frick’s superior collection provided much more evidence than jaw fragments. A facial pocket, known as a fossa, was used to suggest that there were as many as eight different species of “hipparions” living side-by-side in the United States during the Miocene and Pliocene (Table 4.1) (Prothero and Schoch, 2002). However, the “hipparions” were not horse ancestors. They were an extremely successful side branch that migrated into Europe during the Miocene epoch (Table 4.1). This line produced many genera throughout the current United States and Europe, until their extinction during the Ice Age in the Pliocene (Prothero and Schoch, 2002).

The brief periods of time between the World Wars allowed paleontologists to have access to the abundant fossil horses in China. “The Chinese fossils revolutionized the study of hoofed mammals” (Prothero and Schoch, 2002, 197). Besides the discovery of the “Peking Man” in the caves of Zhoukoudian in China, thousands of mammal fossils never before seen were discovered. Prior to the unearthing of fossils in China, most Western paleontologists hit a dead-end when tracing Perissodactyl to a single ancestor. However, the skull of the most primitive Perissodactyl ever known was found in China and dated all the way back to the Paleocene epoch (Table 4.1) (Prothero and Schoch, 2002). Reluctant to group this new fossil as exclusively horse, the paleontologists called the perissodactyl fossil Radinskya, (in honor of the foremost expert on Perissodactyl, Leonard Radinskya) (Prothero and Schoch, 2002). It subsequently came to be accepted after an international conference in the Frick wing of the American Museum of Natural History in December, 1981, that the horse originated in Asia and then spread to Europe and the United States during the Eocene (Table 4.1) as Hyracotherium or Eohippus (Prothero and Schoch, 2002). Refer to Table 4.4 at the end of this section for the evolutionary tree now believed to be the most accurate and logical.